The present invention relates to a rotary vane motor. More particularly, it relates to a rotary vane motor including forced vane expansion for effectively extracting mechanical energy from an expanding, cryogenic gas at low rotational speeds.
Rotary vane motors are well-known and accepted positive displacement machines. The standard rotary vane motor has numerous applications, such as for powering pneumatic wrenches and grinders, or other similar tools. Alternatively, rotary vane motors can be used in a variety of applications requiring forced rotation of a shaft otherwise connected to a separate device, such as a lift gate or cooling fan.
The conventional rotary vane motor typically comprises a housing having a cylindrical interior and a cylindrical rotor eccentrically mounted in an interior of the housing. The rotor, in turn, includes a plurality of uniformly spaced, radially oriented, slots for slidably receiving a plurality of rectangularly shaped vanes. Both the housing and the rotor are typically formed of metal. The eccentric placement of the rotor within the cylindrical enclosure defined by the housing leaves a gap between the rotor and the housing that is crescent-shaped in cross-section. Further, the vanes are designed to reciprocate in their respective slots as their outer edges sealingly and slidably engage the interior surface of the housing.
During use, pressurized fluid (such as compressed air) is admitted at an inlet port in the housing located in close proximity to one of the narrow ends of the crescent-shaped gap. Rotation of the rotor is initialized by the pressurized fluid pushing against the trailing faces of slidable vanes. As a result of the high speed of rotation, the vanes are flung outwards by the centrifugal force such that an outer edge of each of the vanes sealingly engages the inner surface of the housing. Due to the eccentricity of the rotor, the compartments between the vanes become alternately larger and smaller with rotation of the rotor. Finally, the fluid exits the motor at an outlet port located at an opposite end of the crescent-shaped gap. This process is continued, with pressurized fluid acting upon the extended vanes, imparting rotational movement to the rotor. The rotor, in turn, rotates an attached shaft which is otherwise connected to a discrete device, such as a fan.
As previously described, such prior art rotary vane motors are well adapted for powering tools such as pneumatic wrenches and grinders where the required operating speeds of the motor shaft are greater than 2,000 rpm and where a pressurized drive fluid in the form of a supply of compressed and lubricant-containing air is plentifully and cheaply supplied by a shop air compressor. While widely accepted, the standard rotary vane motor design does have certain deficiencies.
In particular, it is recognized that the standard rotary vane motor design results in certain efficiency losses. This loss of efficiency is due to the leakage (or "blow-by") of compressed air between the outer edges of the rotating vanes and the interior wall of the housing. With most applications, however, this loss of efficiency is relatively inconsequential, comprising a relatively small percentage of the overall air mass that flows through the motor at speeds of 2000 rpm or greater. Additional losses can occur due to friction between the vanes and the housing, and more importantly between the vanes and their individual slots. Once again, this friction-created problem is normally inconsequential in machine shop applications as entrained oil or other lubricants typically present in the shop air used to drive such motors keeps the internal friction of the motor down to an acceptable level.
Generally speaking then, the problems associated with the standard rotary vane motor do not rise to a level of great concern with standard shop type applications. However, prior art motor designs are not well-suited for use at relatively low rotational speeds (i.e., under 1500 rpm) or in environments where the pressurized drive fluid contains no lubricant or moisture, such as where it is cryogenically generated. Such an application for a rotary vane motor may occur, for example, in a cryogenic refrigeration system powered by a tank of liquefied carbon dioxide. In such an application, the motor is used to drive an evaporator blower and an alternator to recharge the battery that powers the refrigeration control system, and low rotational speeds are preferred to enhance the efficiency the fan blades of the blower. Because lower volumes of compressed gas are passed through the motor housing at lower speeds below 2000 rpm, the blow-by of gas between the vanes and the side walls of the housing can result in a 20% or greater loss of efficiency in prior art designs, where efficiency is defined as the ability of the motor to convert the energy of the compressed gas into rotary power.
Additionally, the standard rotary vane motor design efficiency decreases when the pressurized fluid used to power the device does not contain lubricants. Once again, this problem is quite prevalent with a cryogenic refrigeration system powered by a tank of liquefied carbon dioxide. Attempts have been made to overcome this friction problem through the use of self-lubricating plastic vanes. While the use of vanes formed from self-lubricated plastic material can ameliorate the frictional problems encountered when the pressurized gas contains no lubricant, the relatively light weight of such vanes can create a sealing problem at low rpm rates utilized in the above-referenced cryogenic refrigeration system. More particularly, the centrifugal force that tends to sling each vane into engagement against the inner surface of the housing may not be of sufficient magnitude to create an effective sealing engagement between the vane and the housing interior.
Other attempts to overcome the frictional problems associated with non-lubricated pressurized fluid have included revising the vane design. Unfortunately, however, it is impossible to eliminate the frictional interaction between the vane and its associated slot. Even more problematic is that liquid carbon dioxide can contain traces of moisture or other viscous impurities. These impurities find their way into the vapor motor and combine with the wear particles from the motor vanes themselves to further amplify frictional effects, resulting in sticking of the vanes. Over time, the motor will no longer start and the system becomes stagnant with dry ice.
Finally, the standard rotary vane motor design loses efficiency under extremes of temperature which can occur, for example, when the drive gas originates from a cryogen such as liquid carbon dioxide. When such a gas is used, the internal components of the motor may be subjected to temperature extremes ranging from -100.degree. F. to +130.degree. F., depending upon the temperature of the refrigerated space and the ambient temperature. Under such conditions, even if the vanes, the rotor slots and the internal dimensions of the housing are carefully dimensioned in order to minimize inefficiencies caused by blow-by, such dimensioning does not hold up over such a broad range of temperature extremes due to the different thermal coefficient of expansions of the different materials forming these components. As a result, the opportunity for binding or sticking of vanes within a respective motor housing is greatly magnified.
Rotary vane motors have proven to be highly useful in a number of applications. However, the inefficiencies associated with the standard design render the rotary vane motor of limited value in certain cases, such as when powered by vaporized liquid carbon dioxide. Therefore, a need exists for a rotary vane motor that is capable of efficiently running at low rpm with cryogenic gas in order to drive certain types of blowers and other devices which operate best at low rpm. In this regard, such a rotary vane type motor should be designed to eliminate the problems associated with friction and other impediments to proper vane movement within an associated slot.
The foregoing illustrates limitations known to exist in prior art methods and apparatus. Thus, it is apparent that it would be advantageous to provide an invention that overcomes the limitations. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.